1. Field of the Invention
This invention relates to pulley structures including two relatively rotatable members that are interconnected so that rotative forces applied to either of the rotatable members are transmitted through a resilient connection to the other of the rotatable members.
2. Background Art
Alternators are provided on automobiles to generate power. The alternator is generally belt driven off of the engine crank shaft. With internal combustion engines, the crank shaft may be repeatedly accelerated and decelerated, whereby variable forces are imparted to an associated belt that drives the alternator.
Generally, alternators have a rotating member/shaft with a large moment of inertia. In the event that there is a change in speed of the crank shaft, a rotating member/pulley, connected to the rotating member/shaft to drive the rotating member/shaft, and driven by the belt, slips relative to the belt so as to thereby generate noise and potentially cause wear on the belt.
Further, the speed variations induced by the crank shaft to the alternator shaft may cause deterioration of the power generating mechanism of the alternator. This may reduce power generating efficiency and eventually lead to a failure.
Consequently, it is preferred that there be some resilient force transmission structure between the rotating member/pulley and rotating member/alternator shaft to absorb the rotational force variations induced by the crank shaft.
Various force transmission structures have been devised and exist in the prior art. For example, it is known to use an elastic member and a viscous fluid acting between two relatively rotating members, as shown in JP-A-08-240246. The pulley structure therein consists of an elastic member made of rubber and a viscous fluid with a viscosity that increases proportionately to an increase in the shear forces generated when a variation in rotation is brought about between the relatively rotating members. Even when the pulley structure is operated with a torque capable of generating a shear stress equal to or larger than the limit of the elasticity of the elastic member, the displacement angle between the rotating members is restrained by the viscous fluid. The elastic member is thereby prevented from being destroyed by yield or rupture.
A rubber element in a ring shape is used as the elastic member between the rotating members. Deformation thereof is dictated by the degree of elasticity for this component. A desired amount of relative angular displacement between the two rotating members may thus not be achievable.
Additionally, while rotational force variations between the crank shaft and alternator can be absorbed by the combined effect of the elastic member and viscous fluid, the belt with this system is subject to being resonated in response to variations in tension thereon, whereby noise may be emitted. This may also have an adverse influence on the durability of the belt.
Another known pulley structure is shown in Japanese Patent Publication No. 3357391. The pulley structure therein is provided only with an elastic member between two relatively rotatable members. The elastic member is a coil spring with a spiral portion at one end, a securing end portion bent to be offset in a radial direction at the other end, and a middle spiral portion therebetween.
The one end portion is provided with an inner diameter capable of being frictionally engaged with a cylindrical surface of one of the rotating members. The other end portion is fit in a groove with a circular arcuate shape in the other rotating member and is secured to the other rotating member by an end portion that is offset in a radial direction.
The rotating members are engaged to be rotated together so long as there is positive torque imparted by movement of an associated belt. The middle spiral portion enables one rotating member to rotate elastically instantaneously in a reverse direction relative to the other rotating member.
Further, in the event that the rotational speed of the crank shaft is reduced to a degree sufficient that a predetermined negative torque is applied between the two rotating members, the spiral portion at the one end, frictionally engaged with a surface of the associated rotating member, experiences a slipping/detaching action resulting from rotating the other rotating member at a speed exceeding the rotational speed of the one rotating member.
A large relative angular displacement between the first rotating members is made possible by the coil spring and therefore a variation in the tension on the belt can be reduced. This in turn reduces noise by reason of belt slippage and improves the durability of the belt. Further, by combining a coupling/detaching mechanism with a one way clutch feature, manufacturing costs may be controlled. The problem of belt resonance, and noise generation and belt deterioration associated therewith, are not as prevalent with this structure as with the previously described prior art structure. In this structure, relative displacement between the rotating members may have an adequate angular range.
However, while the one end portion of the coil spring is contained in the arcuate groove in its associated rotating member, it is locked thereto through the bent end portion. Relative rotational movement of the members resulting from variation in rotational speeds may cause a stress concentration at the bent spring end. There is thus a concern of subjecting the locked, bent end portion of the coil spring to fatigue rupture after repeated stresses generated each time speed variations are encountered.
It is also known to drive an alternator pulley through a belt on a serpentine drive mechanism on an automobile, as shown in Japanese Patent Publication No. 3268007. The alternator has a hub structure rotated together with an armature assembly and an alternating current generator pulley attached to the hub structure. A coil spring is interposed between the hub structure and the alternating current generator pulley and secured by fixing ends thereof to each of the hub structure and alternating current generator pulley. The belt transmits a driving rotational movement through the alternator current generator pulley. The hub structure can be elastically rotated to move relative to the alternating current generator pulley in a reverse direction. The end portions of the coil spring are bent to be offset in a radial direction, with the end portions retained in receptacles provided in the hub structure and the alternating current generator pulley.
Stress concentration at the end portions of the coil fixed to the hub structure and/or the alternating current generator pulley may lead to destruction of the coil spring in a relatively short time period.